Papermaking fabric

ABSTRACT

The invention relates to a papermaking fabric which is designed as a multi-layer woven fabric having an upper fabric layer and a lower fabric layer. The upper fabric layer has a plurality of upper longitudinal threads which run solely in the upper fabric layer and a plurality of upper transverse threads which are interwoven with the upper longitudinal threads and each run at least primarily in the upper fabric layer. The lower fabric layer has a plurality of lower longitudinal threads which run solely in the lower fabric layer and a plurality of lower transverse threads which are interwoven with the lower longitudinal threads and run solely in the lower fabric layer. The upper fabric layer has a repeating upper weave pattern on the upper side thereof which is formed from nine upper longitudinal threads and three upper transverse threads. The three upper transverse threads of the upper weave pattern each have the following course in the upper weave pattern: under a upper longitudinal thread, over the next upper longitudinal thread, under the next upper longitudinal thread, over the next upper longitudinal thread, under the four next successive upper longitudinal threads and over the following upper longitudinal thread, wherein, in the upper weave pattern, the course of the respective upper transverse thread is arranged offset to the course of the adjacent upper transverse thread in the transverse direction by three upper longitudinal threads. At least some of the upper transverse threads in the woven fabric are designed as binding transverse threads which, in order to bind the upper fabric layer to the lower fabric layer, temporarily switch into the lower fabric layer during the course thereof under the four successive to longitudinal threads and there run under at least one lower longitudinal thread.

The present invention relates to a multi-layer paper machine screen, for example to a sheet forming screen of a paper machine, e. g. to a sheet forming screen as it is used in the process of papermaking in the sheet forming section of a wet end of a paper machine for draining/filtration of a fiber suspension and of a paper fiber, respectively. Screens of this type are mainly used for high-quality graphic types of paper having a low paper weight and high requirements concerning printability. These papers can be produced with so-called gap formers at rates of up to 2,000 m/min. In this respect, high demands are made on the screen's mechanical stability, draining performance, fiber support, reduced tendency to marking and lifespan.

A main process in papermaking is the forming of the sheet (=sheet forming) which is effected by draining a fiber suspension or paper fiber by means of filtration in the sheet forming section of the wet end of a paper machine by using a so-called sheet forming screen.

In this respect, the fiber suspension may be understood as a mixture of mechanical pulp or chemical pulp, fillers and auxiliary chemical agents suspended in water.

In order to be able to produce a paper sheet as uniform as possible, it is necessary to increase or set the amount of water within the fiber suspension immediately before the sheet formation to approximately 99%. This ensures that the fibers can be distributed uniformly in the water, which is beneficial to the quality of the sheet to be formed.

The amount of water is reduced to approximately 80% by the above-mentioned filtration process within the sheet forming section, i.e. during the sheet forming process. The paper fibers and the fillers and auxiliary agents remain on the paper machine screen in a uniformly distributed manner in the form of a nonwoven fabric.

While in the past the draining process took place mainly by means of a paper machine screen applied to a Fourdrinier paper machine, mainly twin screening machines are being used these days, for example in the form of so-called gap formers. These twin screening machines are characterized in that the fiber suspension is injected into a gap which is formed between two paper machine screens, so that draining can take place simultaneously through both screens, whereby it is possible to significantly accelerate the filtration process and thus also the production rate of the papermaking machine. There are papermaking machines these days for types of paper having a low grammage, which are capable of producing with speeds of more than 2,000 m/min.

The extreme requirements for the paper to be produced and the conditions existing in the papermaking machine require sheet forming screens which are designed e. g. for this purpose and which offer high fiber support, high openness and a high mechanical stability at the same time. In addition, a low tendency to marking of the fabric, i.e. a high uniformity of the paper forming screen is necessary e. g. for the domain of graphic papers.

Multi-layer paper machine screens have proven of value for these fields of application over the past years, comprising two sides formed in a different manner, which are adapted to the particular purpose of use. Screens of this type have a paper side which is formed by the upper side of an upper fabric. In habitual language use, the paper side is also referred to as the upper side of the screen, and is relevant for forming the paper sheet. In addition, these screens have a machine side which is formed by the lower side of a lower fabric. The machine side which can also be referred to as the lower side of the screen contacts the members of the papermaking machine. The respective screen side has a machine direction and a cross direction. In this respect, machine direction (also MD for “machine direction”) refers to the running direction of the paper web and therefore also to the running direction of the paper machine screen, and the cross direction (also CMD for “cross machine direction”), sometimes also referred to as cross machine direction, is the direction turned by 90° in the plane of the paper machine screen, i.e. the direction located transverse to the running direction of the paper and the screen.

Due to the very specific configuration of modern paper machine screens, usually neither the paper and machine side nor the machine and cross direction are interchangeable, as otherwise the mode of operation of the screen would not be ensured or would not be ensured sufficiently. For example, the machine direction threads (=longitudinal threads) on the machine side which realize circulation of the screen, can be protected against wear by transverse threads projecting or protruding significantly. Providing a balanced relationship of longitudinal and transverse threads on the paper side can ensure a good depositing possibility for the paper fibers. With respect to the fiber support, but also with respect to the tendency to marking of the screen, the most simple and at the same time the oldest basic weave of textile engineering has proven of value for the upper fabric and thus for the paper side, namely the so-called plain weave. In this kind of weave, the repeat (=the smallest repeating unit of the weave) of which is formed exactly by two warp threads (as a general rule, the longitudinal threads/machine direction threads of the screen are formed by the warp threads) and two weft threads (as a general rule, the transverse threads of the screen are formed by the weft threads), the threads are connected to a fabric in a particularly close and uniform manner. Although the plain weave is very well suited for forming a paper sheet and is hence very well suited for the paper side, it is usually not suited very well for the machine side. If a paper machine screen is provided with a plain weave paper side, it can therefore be advisable to provide for a second fiber layer underneath the plain weave, forming the machine side of the screen, which gives the screen sufficient stability and wearing potential.

In this respect, the connection of the two layers (i.e. of the upper fabric forming the paper side and the lower fabric forming the machine side) is a particular challenge, amongst others due to the fact that the plain weave favorable for the paper side involves particularly unfavorable preconditions for such a layer connection.

The state of the art describes different approaches for connecting two screen fabric layers, one approach of which describes the use of additional, separate binder threads extending in a longitudinal direction or a transverse direction. According to this approach, two finished and completed fabric layers are connected to each other by separate, fabric-external binder threads, which binder threads do not contribute to/are not required for establishing the corresponding fabric layer weave. Both fabric layers consist of longitudinal threads and transverse threads which extend exclusively in the respective fabric layer and thereby completely generate the respective fabric layer pattern or fabric layer weave. This approach is, for example, described in CA 1 115 177 A1, where separate binder weft threads are used which bind with warp threads of the upper fabric and warp threads of the lower fabric, and in DE 39 28 484 A1 in which separate warp threads are used as binder threads. Other examples can be found in DE 42 29 828 A1, WO 93/00472, and EP 0 136 284 A2. The separate binder threads are usually configured to be thinner than the threads forming the respective fabric layer (cf., for example, CA 1 115 177 A1), as the binder threads must be incorporated in the fabric structure in addition to the fabric forming threads. In this respect, little space is provided for such separate binder threads e. g. in a plain weave. Otherwise, the binder threads would interfere with the originally homogeneous structure of the weave, so that imperfections which cause markings in the paper would be produced e. g. in the plain weave provided on the paper side. However, practice has shown that the thin binder threads wear out and break rather fast e. g. in paper machines which process a high amount of abrasive fillers or the construction of which puts heavy bending loads on the screens in the machine direction, so that the two fabric layers are first displaced and then separated as a result thereof. It goes without saying that it is impossible to make high quality paper by means of a fabric/screen changed in such a manner.

An alternative is the use of the fabric's own threads for connecting the layers. In this respect, the threads used for connecting the layers on the one hand serve for connecting the layers, for which purpose they alternate between the layers, and, on the other hand, also form the upper fabric or the lower fabric. Different threads of the fiber may be used as connecting threads, i.e. transverse threads and/or longitudinal threads of the upper fabric and/or of the lower fabric; in this respect, the different binding threads bring about different screen properties.

In addition, it is known to use two transverse threads which interact as a so-called functional transverse thread pair. Either one or both of the transverse threads of a functional pair extend alternately in the upper fabric and the lower fabric. In this respect, both transverse threads of a functional pair can form a virtually uninterrupted transverse thread of a paper side plain weave, i.e. an upper composite transverse thread. Those thread portions of the functional pair which are currently not required for forming the virtually uninterrupted transverse thread on the paper side extend in the interior of the fabric and can be used for binding the lower fabric to the upper fabric. In this respect, the thread portion binding the lower fabric can, for example, complete the lower fabric or its weave at the same time. For example, an upper transverse thread may be provided between two functional transverse thread pairs, which completes exclusively the plain weave (i.e. which extends only in the upper fabric), but has no binding function. Exemplary embodiments of this approach can be found, for example, in EP 0 097 966 A2, EP 794 283 A1, WO 99/06630 A1, WO 99/06632 A1, and WO 02/14601 A1.

Alternatively, the layers may be connected by so-called functional longitudinal thread pairs. EP 0 069 101 and EP 093 096 are pointed out as examples in this regard, showing a layer connection through functional longitudinal thread pairs.

An object of the present invention may be seen in providing a sheet forming screen made of a multi-layer fabric, which meets the requirements described above at least in part, e. g. completely, i.e. for example, comprises a high fiber support, a low tendency to marking, an appropriate mechanical stability and a stable layer connection.

To achieve this object, the invention provides a paper machine screen according to claim 1. Further embodiments of the screen according to the invention are described in the dependent claims.

The screen according to the invention is formed of an upper transverse thread-bound, e. g. upper weft-bound, multi-layer fabric, i.e. of a multi-layer fabric, the upper layer of which is (e. g. exclusively) connected to the lower layer by means of upper transverse threads. This means that the fabric used for the screen forms part of the group of fabrics, the layers of which are connected by means of the fabric's own threads. The connecting upper transverse threads contribute, on the one hand, to the layer connection and, on the other hand, to the completion/formation of the upper fabric layer or binding weave thereof.

The screen according to the invention has a high degree of fineness on the paper side in order to provide an appropriate fiber support. Due to the uniform design of the paper side, the tendency to marking is low. The machine side may have an increased openness for good draining properties and a reduced tendency to congestion of the fabric with fibers and impurities when compared to the paper side. The machine side has, e. g., a high mechanical stability against expansion/elongation. The internal wear and in the end a layer separation may be avoided or severely reduced in the multi-layer sheet forming screen, the fabric layers of which are connected by upper transverse threads.

According to the invention, a fine fabric with a very homogeneous design may therefore be used for sheet forming and may be bound to a stable and coarser lower fabric. The upper fabric is realized in a weave that is comparable to a plain weave and is therefore optimally suited for the production of graphic paper. E. g., the lower fabric is configured to be coarser in longitudinal and transverse directions, and hence provides the following advantages: solid matter potentially having entered the fabric through the upper fabric is not kept in the lower fabric; the draining performance of the screen is exclusively determined by the paper side; the lower fabric is open to such a degree that its influence on the total flow is subordinate. E. g., the coarse lower fabric is used for optimizing the entire screen with respect to mechanical stability and resistance to wear and fabric thickness.

According to the invention, the paper side fabric is thus configured to be relatively fine and homogeneous. The machine side fabric layer may be configured to be more robust and to have a higher degree of openness, e. g. with relatively thick warp threads and weft threads which contribute to a higher mechanical stability. There is a stable connection of the upper and lower fabrics by a specific design of the paper side transverse threads with a binding zone/area. Depending on the application, this may help to set an appropriate weave frequency.

The paper machine screen according to the invention has an upper fabric layer which is formed by a plurality of upper longitudinal threads running solely in the upper fabric layer, and a plurality of upper transverse threads interwoven with the upper longitudinal threads and each running either entirely or predominantly in the upper fabric layer. The interwoven upper transverse threads and upper longitudinal threads together form an upper weave, i.e. an overlapping pattern of the upper fabric layer. The upper pattern repeat (see below), a pattern constantly repeating on the paper side, which reproduces the course of the upper transverse threads with respect to the upper longitudinal threads is characteristic of this upper weave.

Moreover, the paper machine screen according to the invention has a lower fabric layer which is formed by a plurality of lower longitudinal threads running exclusively in the lower fabric layer, and a plurality of lower transverse threads interwoven with the lower longitudinal threads and running exclusively in the lower fabric layer. The interwoven lower transverse threads and lower longitudinal threads together form a lower weave, i.e. an overlapping pattern of the lower fabric layer. The lower pattern repeat, a pattern repeating constantly on the machine side, is characteristic of this lower weave.

The lower longitudinal threads may be greater in diameter than the upper longitudinal threads, and the lower transverse threads may be greater in diameter than the upper transverse threads. The lower side of the screen contacting the paper machine may thereby be designed to be robust, whereas the upper side of the screen contacting the fiber suspension may be designed to be fine. If the lower longitudinal threads and the lower transverse threads do not change into the upper layer, the fine paper side is not interfered with by the robust lower threads. The comparatively thin upper transverse threads which change into the lower layer, interfere with the lower weave only insignificantly. Moreover, the comparatively thick lower transverse threads project further downward than the upper transverse threads during their stay in the lower layer, whereby the upper transverse threads are shielded by the lower transverse threads.

The ratio of upper longitudinal threads to lower longitudinal threads in the entire fabric is, for example, greater than or equal to 1, for example greater than 1. For example, the ratio may be 3:2 or alternatively 1:1. The ratio of 3:2 can be useful in this regard, favoring the formation of a fine upper side and a coarse lower side. Moreover, the lower transverse threads can be configured to be particularly thick with this ratio; nevertheless, a sufficiently open lower side can be achieved in this regard.

The fabric may, for example, have a repeating total repeat in which the ratio of upper longitudinal threads to lower longitudinal threads is 9:6 or 9:9.

The ratio of upper transverse threads to lower transverse threads in the entire fabric may, for example, be greater than 1, for example 2:1. The formation of a fine upper side and a coarse lower side is favored thereby as well.

The fabric may, for example, have a repeating total repeat in which the ratio of upper transverse threads to lower transverse threads is 6:3 or 12:6.

The fabric may, for example, have a repeating total repeat in which the ratio of upper longitudinal threads to upper transverse threads is 9:12 or 9:9 or 9:6. This represents in each case a balanced ratio of upper longitudinal threads to upper transverse threads, with which a fine upper side can be formed.

According to the invention, the respective upper pattern repeat comprises nine upper longitudinal threads and three upper transverse threads. Each of the three upper transverse threads of a respective upper pattern repeat has the following course: under an upper longitudinal thread, over the next upper longitudinal thread, under the next upper longitudinal thread, over the next upper longitudinal thread, under the four next successive upper longitudinal threads and over the following upper longitudinal thread. In this respect, the course (and the starting point, respectively) of the respective upper transverse thread is arranged to be offset to the course of the adjacent upper transverse thread in a transverse direction by three upper longitudinal threads.

The upper transverse threads repeat the described course in a transverse direction constantly and immediately one after another, so that the respective upper transverse thread is interwoven with the upper longitudinal threads of the fabric in the manner of a plain weave with a so-called binding area/zone being formed regularly, in which the upper transverse thread extends between the fabric layers or in the lower fabric layer, depending on whether it is or is not formed as a binding upper transverse thread. In this respect, the respective binding area/zone extends over a distance of four upper longitudinal threads, and five upper longitudinal threads are arranged between two adjacent binding areas/zones of an upper transverse thread.

At least some of the upper transverse threads are formed as connecting transverse threads for connecting the layers in the fabric. For this purpose, the upper transverse threads comprise a so-called binding area which is formed by the thread portion extending underneath the four upper longitudinal threads. For connecting the upper fabric layer with the lower fabric layer, some or all upper transverse threads may temporarily change into the lower fabric layer during their course under the four successive upper longitudinal threads, i.e. in the binding area/zone, and there extend at least under one lower longitudinal thread, in order to thereby bind the lower longitudinal thread and hence the lower fabric to the upper fabric.

Hence, at least one of the upper transverse threads should be formed as a connecting transverse thread in the respective total repeat of the fabric (repeating weave pattern of the entire fabric, formed by upper longitudinal and transverse threads and lower longitudinal and transverse threads). In this respect, a connecting upper transverse thread of this type repeatedly connects one or more lower longitudinal threads to the upper fabric layer along its course through the fabric in a transverse direction (in each binding zone), whereby the two layers are connected with each other.

For example, each lower longitudinal thread may be bound to the upper fabric layer at least once, i.e. for example exactly once (see the second embodiment according to FIG. 5) or exactly twice (see the second embodiment according to FIG. 6) in the respective total repeat of the fabric. In this respect, each binding may be performed by a separate upper transverse thread. If each lower longitudinal thread is bound at least once to the upper fabric layer in the respective total repeat of the fabric by means of the upper transverse threads, a stable layer connection and therefore a long lifetime of the screen can be achieved.

However, there may also be upper pattern repeats, the upper transverse threads of which are all formed as non-connecting upper transverse threads. This, however, reduces the number of layer connections in the respective total repeat.

It may be useful for many applications that at least one of the three upper transverse threads of each upper pattern repeat is formed as a connecting transverse thread. E. g., at least two, e. g. each of the three upper transverse threads of each upper pattern repeat may be formed as a connecting transverse thread.

In this respect, the number of layer connections in the respective total repeat may be set appropriately depending on the requirements and the application, respectively.

The respective connecting upper transverse thread may extend under exactly one lower longitudinal thread or exactly two successive lower longitudinal threads during its course under the four successive upper longitudinal threads for connecting the upper fabric layer with the lower fabric layer.

The connection of the upper fabric layer and the lower fabric layer can be performed exclusively with the above-described upper connecting transverse threads. This means that no separate binding threads are required for keeping the fabric together.

Therefore, the upper fabric layer may solely consist of a plurality of adjoining upper pattern repeats. I.e., no other threads are arranged in this case between the individual upper pattern repeats. Furthermore, the respective upper pattern repeat may solely consist of the nine upper longitudinal threads and the three upper transverse threads.

The lower transverse threads may be interwoven with the lower longitudinal threads in a manner to form a 3-shaft or 6-shaft weave in the lower fabric layer, in which the course of the respective lower transverse thread is constantly repeated after three and six lower longitudinal threads, respectively. These are examples by means of which a coarse and open machine side can be achieved respectively. In the 3-shaft weave, the respective lower transverse thread may, for example, first extend over a lower longitudinal thread and then under the two following lower longitudinal threads, and the respective lower transverse thread may, in the 6-shaft weave, extend for example first over a lower longitudinal thread and then under the five following lower longitudinal threads. If the respective total repeat comprises nine lower longitudinal threads, the lower fabric layer may, for example, be configured with three shafts or with nine shafts.

E. g., the longitudinal threads are formed as warp threads and the transverse threads are formed as weft threads.

Further variations of the screen according to the invention can be derived from the following description of exemplary embodiments.

Hereinafter, some of the terms used in this application shall be defined:

Longitudinal threads are threads of the screen/fabric which extend in the longitudinal direction or longitudinal extension of the screen and are arranged in the running direction of the paper machine in operation. In the flat woven screen, the longitudinal threads are formed by the warp threads of the weaving loom. Circular woven fabrics, in contrast, realize the longitudinal threads by means of wefts.

Transverse threads are threads of the screen/fabric which extend in a transverse direction of the screen and are arranged transverse to the running direction of the paper machine in operation. In the flat woven screen, the transverse threads are formed by the wefts. Circular woven fabrics, in contrast, realize the transverse threads by means of the warps of the weaving loom.

A fabric layer is a single-layer fabric consisting of interwoven transverse threads and longitudinal threads (or warps and wefts).

The upper fabric or the upper fabric layer is a fabric layer which is usually formed in a particularly fine manner, which usually forms the paper side (=the upper side of the upper fabric oriented outwards) of the screen, on which the paper fiber layer is formed. The upper layer is located on the “logical upper side” of the screen.

The lower fabric or the lower fabric layer is a fabric layer which is usually formed in a particularly robust manner, which usually forms the machine side (=the lower side of the lower fabric oriented outwards) of the screen, which comes in direct contact with the driving and draining members of the paper machine generating wear.

Upper longitudinal threads are threads which are located exclusively in the upper fabric and which are there interwoven with transverse threads extending in the upper fabric. Upper longitudinal threads never leave the upper fabric, i.e. they do not change into the lower fabric.

Upper transverse threads are threads which are at least predominantly located in the upper fabric and which are there interwoven with the upper longitudinal threads. Upper transverse threads within the meaning of this application may therefore either extend exclusively in the upper fabric or may temporarily change into the lower fabric. At least a part of the upper transverse threads changes into the lower fabric, so as to bind it to the upper fabric.

Lower longitudinal threads are threads which are located exclusively in the lower fabric and which are there interwoven with transverse threads extending in the lower fabric. The lower longitudinal threads never leave the lower fabric, i.e. they do not change into the upper fabric.

Lower transverse threads are threads which are located exclusively in the lower fabric and which are there interwoven with the lower longitudinal threads. The lower transverse threads never leave the lower fabric, i.e. they do not change into the upper fabric.

The pattern repeat of the upper fabric or the so-called upper pattern repeat is a recurring pattern or a repeating unit in the upper fabric of interwoven upper longitudinal threads and upper transverse threads, e. g. the smallest repeating unit in the upper fabric. In a plan view onto the upper fabric or the paper side of the screen, a plurality of such upper pattern repeats can be seen in the longitudinal and transverse directions of the screen, and are, e. g. in the transverse direction (e. g. also in the longitudinal direction) strung directly together. Hence, the upper pattern repeat may represent the recurring overlapping pattern of the upper fabric formed in the top view of the upper fabric by the upper longitudinal threads and upper transverse threads. If one of the upper transverse threads temporarily changes into the lower fabric, the exact course of the upper transverse thread in the lower fabric has no influence on the upper pattern repeat, as the upper transverse thread always extends under the upper longitudinal threads during this time. In other words, the upper pattern repeat regards the course of the upper transverse threads with respect to the upper longitudinal threads and the overlapping pattern resulting therefrom; the course of the upper transverse threads with respect to the lower longitudinal threads is of no importance for determining the upper pattern repeat.

A pattern repeat of the lower fabric or a so-called lower pattern repeat may be defined/specified analogously.

The total repeat of the fabric is a recurring weave pattern/overlapping pattern of the entire fabric, e. g. the smallest repeating unit of the entire fabric, the course of all threads (upper and lower longitudinal threads, upper and lower transverse threads) being taken into consideration. Knowing the total repeat, the complete fabric or screen may thus be produced. I.e., the screen or fabric may consist of a plurality of total repeats directly strung together.

A longitudinal thread repeat is the smallest repeating unit of longitudinal threads in the entire fabric.

A transverse thread repeat analogously refers to the smallest repeating unit of transverse threads in the entire fabric.

The invention will hereinafter be described in more detail by means of different exemplary embodiments and with reference to the drawing. In the drawing:

FIG. 1 shows a top view of a schematically illustrated complete upper pattern repeat of a screen according to the invention,

FIG. 2 shows a photograph showing an enlarged view of a detail of the paper side of a screen according to the invention, which consists of a plurality of adjoining upper pattern repeats, i.e. a top view of the upper side of a screen according to the invention or the upper/paper side fabric layer thereof, the lower/machine side fabric layer having been omitted/cut out for the purpose of clarity,

FIG. 3 shows the photograph according to FIG. 2, an individual upper pattern repeat (continuous lines) and the eight upper pattern repeats surrounding the same (broken lines) having been highlighted,

FIG. 4 shows different alternative courses for the three upper transverse threads of the upper pattern repeat according to FIG. 1 within the entire fabric, i.e. with respect to the upper and lower fabric layers/longitudinal threads,

FIG. 5 shows a schematic illustration of the total repeat of a screen according to a first embodiment of the invention, and

FIG. 6 a schematic illustration of the total repeat of a screen according to a second embodiment of the invention.

FIG. 1 shows a schematic top view of a complete upper pattern repeat of a screen according to the invention. This upper pattern repeat recurs on the upper side of the screen, i.e. the upper side of the screen comprises a plurality of such identically formed upper pattern repeats.

As shown by FIG. 2 and FIG. 3, the paper side of a screen according to the invention may, for example, consist of a plurality of such upper pattern repeats which are arranged to directly abut on one another. I.e., eight directly abutting, identically formed upper pattern repeats are arranged around the upper pattern repeat shown in FIG. 1 on the paper side of the screen (at the four corners as well as above, underneath, on the right and on the left).

In FIGS. 1 to 3, the longitudinal threads extend vertically and the transverse threads extend horizontally.

The upper pattern repeat consists of 9 longitudinal threads (1, 2, 3, . . . , 9) and 3 transverse threads (21, 22, 23). In FIG. 3, the longitudinal threads and the transverse threads of a neighboring repeat are provided with one or two “'”.

In FIG. 1, each square represents a crossing of a longitudinal thread (for example a warp thread) and a transverse thread (for example a weft thread), as it is common practice in textile engineering. If the square is provided with a cross, the longitudinal thread extends above the transverse thread, and accordingly the transverse thread extends under the longitudinal thread; otherwise, the longitudinal thread extends under the transverse thread. The circle in FIG. 3 indicates that the longitudinal thread extends under the transverse thread and accordingly the transverse thread extends above the longitudinal thread.

As shown in FIG. 1, the courses of the three upper transverse threads 21 to 23 are basically equal in the upper pattern repeat with respect to the nine upper longitudinal threads 1 to 9. E. g., each upper transverse thread 21 to 23 first extends under an upper longitudinal thread, then above the next upper longitudinal thread, then under the next upper longitudinal thread, then above the next upper longitudinal thread, then underneath four successive upper longitudinal threads, and finally above the next upper longitudinal thread.

For example, the upper transverse thread 21 in FIG. 1 first extends under the upper longitudinal thread 1, then above the upper longitudinal thread 2, then under the upper longitudinal thread 3, then above the upper longitudinal thread 4, then under the four successive upper longitudinal threads 5 to 8, and finally above the upper longitudinal thread 9. Then, the transverse thread 21 repeats this course. I.e., the upper transverse thread 21 repeats the described course in the upper pattern repeat not shown in FIG. 1, directly on the right next to the upper pattern repeat shown, and so on; see e. g. FIG. 3.

The upper transverse thread 22 has the same course, but its course or rather the starting point thereof is offset to the right by three upper longitudinal threads with respect to that of the upper transverse thread 21. In this respect, the above-described course (once underneath, once above, once underneath, once above, four times underneath, once above) starts at the upper longitudinal thread 4 for the upper transverse thread 22, i.e. the upper transverse thread 22 first extends under the upper longitudinal thread 4, then above the upper longitudinal thread 5, then under the upper longitudinal thread 6, then above the upper longitudinal thread 7, then underneath the four successive upper longitudinal threads 8, 9, 1, 2, and finally above the upper longitudinal thread 3. It should be noted that it is common usage in textile engineering to count “over edge/corner” for determining the thread's course in a repeat. This is insofar justified, as another repeat directly abuts in a transverse direction against the repeat shown.

The upper transverse thread 23 also has the above-described course (once underneath, once above, once underneath, once above, four times underneath, once above), but its course is also offset by three upper longitudinal threads to the right with respect to the neighboring upper transverse thread 22. In this respect, the above-described course starts at the upper longitudinal thread 7 for the upper transverse thread 23, i.e. the upper transverse thread 23 first extends under the upper longitudinal thread 7, then above the upper longitudinal thread 8, then under the upper longitudinal thread 9, then above the upper longitudinal thread 1, then underneath the four successive upper longitudinal threads 2 to 5, and finally above the upper longitudinal thread 6.

In summary, the course of the respective upper transverse thread in the upper pattern repeat is arranged to be offset by three upper longitudinal threads in a transverse direction with respect to the course of the neighboring upper transverse thread; in textile engineering, this is usually referred to as pitch. I.e., the pitch of the upper transverse threads in the upper pattern repeat and accordingly on the paper side is three according to the invention.

As already mentioned, the paper side of the screen according to the invention is made of a plurality of upper pattern repeats according to FIG. 1. This is illustrated by FIG. 2 and e. g. by FIG. 3 in which one of the upper pattern repeats is marked with a rectangle of continuous lines. The eight upper pattern repeats directly abutting against this upper pattern repeat are represented by broken rectangles, six of the eight abutting upper pattern repeats in FIG. 2 and FIG. 3 being illustrated only in part. Other than the “X” in FIG. 1, the “◯” in FIG. 3 indicates that the upper transverse thread extends over the upper longitudinal thread (and accordingly, that the upper longitudinal thread extends under the upper transverse thread).

As can also be seen clearly in FIGS. 2 and 3, the upper longitudinal threads are arranged next to each and substantially in parallel and spaced with respect to one another. The upper transverse threads are also arranged next to each other and substantially in parallel, and thus do not form any functional pairs. The upper transverse threads may basically be spaced with respect to each other as well, but may contact each other in sections.

According to the invention, at least some of the upper transverse threads in the fabric are formed as connecting transverse threads. Connecting transverse thread serve as a connection of the upper fabric layer and the lower fabric layer. According to the invention, this connection occurs during the course of an upper transverse thread underneath the four successive upper longitudinal threads. In this respect, the upper transverse thread temporarily changes into the lower fabric layer, in order to extend under one or more lower longitudinal threads there and to thereby bind them/it to the upper fabric.

In other words, the screen according to the invention is a so-called weft-bound or rather transverse thread-bound multi-layer fabric, e. g. a purely weft-bound or rather transverse thread-bound multi-layer fabric, i.e. a fabric with no further connecting threads.

FIG. 4 shows alternative embodiments or rather courses of the upper transverse threads 21 to 23 from FIG. 1 within the entire fabric—i.e. with respect to the upper and lower fabric layers. The course of an upper transverse thread is respectively shown over a distance of nine upper longitudinal threads; the respective course recurs constantly and directly in the fabric in a transverse direction, i.e. the transvers thread 21 is, for example, made of a plurality of directly successive thread portions, respectively corresponding to the course 21 a or alternatively respectively corresponding to the course 21 b or alternatively respectively corresponding to the course 21 c or alternatively respectively corresponding to the course 21 d.

The courses of the threads 21 a to 21 d with respect to the upper fabric and accordingly with respect to the upper longitudinal threads are formed to be identical and correspond to the above-described course of the thread 21 (including the starting point). The course of the threads 22 a to 22 d with respect to the upper fabric analogously corresponds to the course of the thread 22, etc.

In this respect, threads marked with the index “a” are formed as pure upper transverse threads which extend exclusively in the upper fabric and do not contribute to a layer connection.

The threads marked with the index “b”, “c” or “d”, on the other hand, are formed as connecting transverse threads which contribute to a connection of the layers.

The threads with the index “b” and the index “c” bind within a so-called binding area/ zone which is formed by the transverse thread portion extending under the four successive upper longitudinal threads exactly one lower longitudinal thread to the upper fabric layer or rather extend thereunder. In this respect, the threads with the index “b” bind in a transverse direction—as viewed from the left to the right—with the respectively first lower longitudinal thread 13, 15 and 11, respectively. I.e., these threads change in the binding area/zone immediately downward into the lower fabric layer, in order to bind the first possible lower longitudinal thread to the upper fabric. The threads with the index “c”, on the other hand, bind in a transverse direction—as viewed from the left to the right—with the respectively second lower longitudinal thread 14, 10 and 12, respectively; i.e., these threads extend above the first possible lower longitudinal thread and bind only the following lower longitudinal thread to the upper fabric.

The threads with the index “d” bind exactly two lower longitudinal threads to the upper fabric layer or rather extend thereunder within the binding area/zone.

The upper transverse thread courses shown in FIG. 4 may be combined with each other in any way for a respective upper pattern repeat and accordingly for three upper transverse threads adjacent in a longitudinal direction, as long as exactly one thread is chosen from each group; i.e. any thread from the first group (21 a to 21 d) may be combined with any thread of the second group (22 a to 22 d) and any thread of the third group (23 a to 23 d). For example, one or more of the upper pattern repeats of FIG. 3 and accordingly three upper transverse threads of FIG. 3 adjacent in a longitudinal direction may be formed by the threads 21 c, 22 c, 23 c, or by the threads 21 b, 22 b, 23 b or by the threads 21 a, 22 a, 23 a (it is also possible that one or more upper pattern repeats, but not all, are exclusively formed by non-connecting upper transverse threads) or by three threads 21 a, 22 b, 23 c.

As already mentioned, at least some of the upper transverse threads in the fabric are formed as connecting transverse threads according to the invention. Consequently, at least some of the upper pattern repeats, for example all upper pattern repeats, comprise one or more connecting upper transverse threads “b”, “c” or “d”.

In the respective total repeat, the respective upper pattern repeat may therefore be composed in any way, as long as exactly one thread is chosen from each group “21”, “22”, “23”; in this respect, the upper pattern repeats may be composed differently (see, for example, FIGS. 5 and 6), and at least one of the upper pattern repeats comprises at least one connecting upper transverse thread, so that in the fabric there are several connecting threads and accordingly in the total repeat of the fabric there is at least one connecting thread.

Having said that, two embodiments of the invention shall now be described by way of example.

The total repeat (i.e. a complete pattern repeat of the fabric) of a screen according to a first embodiment of the invention shall now be described with reference to FIG. 5. Thus, a screen according to the invention may be generated by arranging next to each other several total repeats of this type and accordingly may consist of a plurality of total repeats of this type.

The total repeat shown in FIG. 5 has nine upper longitudinal threads 1 to 9 and six lower longitudinal threads 10 to 15 which are here each formed as warp threads and extend exclusively in the upper and the lower fabric layer, respectively. The upper warp threads 1 to 9 are configured to be thinner than the lower warp threads 10 to 15. The warp or longitudinal thread ratio of paper side/upper fabric to machine side/lower fabric in the total repeat and in the fabric is accordingly 9:6 and 3:2, respectively.

Moreover, the total repeat shown in FIG. 5 comprises twelve upper transverse threads 21 to 23 and six lower transverse threads 31 to 36. The transverse threads are here each formed as weft threads. The lower transverse threads 31 to 36 extend exclusively in the lower fabric layer. The upper transverse threads each change between the two fabric layers, i.e. they are all formed as connecting transverse threads but mainly extend in the upper fabric layer and there form the upper fabric and its weave together with the upper longitudinal threads. The upper weft threads are formed to be thinner than the lower weft threads 31 to 36. The weft or transverse thread ratio of paper side/upper fabric to machine side/lower fabric in the total repeat and in the fabric accordingly is 12:6 and 2:1, respectively.

In the total repeat, each upper transverse thread 21 to 23 has, with respect to the upper longitudinal threads 1 to 9, the following course described with respect to FIG. 1: once underneath, once above, once underneath, once above, four times underneath, once above. The transverse thread pitch in the total repeat is three. The respective transverse thread course constantly recurs in the fabric in a transverse direction. In the fabric, each upper transverse thread forms together with the upper longitudinal threads a plain weave (once underneath, once above, etc.) on the paper side, which is interrupted along each upper transverse thread repeatedly in sections along a distance of four upper longitudinal threads by forming a binding area/zone.

In the total repeat, the twelve upper transverse threads form four upper pattern repeats a to c, d to f, g to i, j to 1 together with the nine upper longitudinal threads. The three upper transverse threads 21, 22, 23 of the upper pattern repeat a to c and the three upper transverse threads 21, 22, 23 of the upper wave repeat g to i are each formed as “b” threads in this regard, which in their binding area/zone extend under the first of two lower longitudinal threads and thereby bind it to the upper fabric. The three upper transverse threads 21, 22, 23 of the upper pattern repeat d to f and the three upper transverse threads 21, 22, 23 of the upper pattern repeat j to k are each formed as “c” threads, which in their binding area/zone extend under the second of two lower longitudinal threads and thereby bind it to the upper fabric.

The course of the transverse threads in the fabric portion a to f is identical to the course of the transverse threads in the fabric portion g to 1. Thus, the twelve upper transverse threads form two upper transverse thread repeats a to f, g to 1 with the nine upper longitudinal threads.

According to the first embodiment, the six lower transverse threads 31 to 36 form a relatively open and coarse lower fabric or rather a relatively open and coarse machine side together with the six lower longitudinal threads 10 to 15. The six lower transverse threads 31 to 36 and the six lower longitudinal threads 10 to 15 form exactly one lower pattern repeat in the total repeat. In the lower pattern repeat, every lower transverse thread extends under five successive lower longitudinal threads and then extends above a lower longitudinal thread; see for example the lower longitudinal thread 36. Every lower transverse thread extends above another lower longitudinal thread. Thus, the lower fabric is formed with 6 shafts in a transverse direction here, i.e. the course of the respective lower transverse thread recurs after six longitudinal threads.

In the embodiment shown in FIG. 5, the lower fabric is bound to the upper fabric twelve times within one total repeat, namely once by each upper transverse thread. In this respect, every lower longitudinal thread is bound twice, namely by two different upper transverse threads. Due to the fact that the upper transverse threads are formed to be thinner than the lower transverse threads in this case, the lower weave (or rather the overlapping pattern formed by the lower transverse and longitudinal threads) is not substantially interfered with by the upper transverse threads temporarily immerging in the lower fabric. Moreover, the lower transverse threads shield the upper transverse threads. Hence, both a stable layer connection and a fine, homogeneous paper side can be formed at the same time.

The total repeat of a screen according to a second embodiment of the invention shall now be described with reference to FIG. 6. A screen according to the invention may consist of a plurality of total repeats of this type.

The total repeat shown in FIG. 6 has nine upper longitudinal threads 1 to 9 and six lower longitudinal threads 10 to 15, which are respectively formed as warp threads and extend exclusively in the upper and lower fabric layers, respectively. The upper longitudinal threads 1 to 9 are formed to be thinner than the lower longitudinal threads 10 to 15. The warp or longitudinal thread ratio of upper fabric to lower fabric in the total repeat and in the fabric is thus again 9:6 and 3:2, respectively.

Moreover, the total repeat shown in FIG. 6 comprises six upper transverse threads 21 to 23 and three lower transverse threads 41 to 43. The transverse threads are again formed as weft threads. The lower transverse threads 41 to 43 extend exclusively in the lower fabric layer. The upper transverse threads each change between the two fabric layers, i.e. they are all formed as connecting transverse threads but each extend mainly in the upper fabric layer. The upper transverse threads are formed to be thinner than the lower transverse threads 41 to 43. The weft or transverse thread ratio of paper side to lower fabric in the total repeat and in the fabric accordingly is 6:3 and 2:1, respectively in this case.

Each upper transverse thread 21 to 23 has, with respect to the upper longitudinal threads 1 to 9, the following course described with reference to FIG. 1: once underneath, once above, once underneath, once above, four times underneath, once above. The transverse thread pitch in the total repeat or rather in the fabric is again three.

In the total repeat, the six upper transverse threads form two upper pattern repeats a to c, d to f together with the nine upper longitudinal threads. The three upper transverse threads 21, 22, 23 of the upper pattern repeat a to c are each formed as “b” threads in this regard, and the three upper transverse threads 21, 22, 23 of the upper pattern repeat d to f are each formed as “c” threads in this regard.

According to the second embodiment, the three lower transverse threads 41 to 43 form a relatively open and coarse lower fabric together with the six lower longitudinal threads 10 to 15. The three lower transverse threads 41 to 43 and the six lower longitudinal threads 10 to 15 form exactly one lower pattern repeat in the total repeat. In the lower pattern repeat, every lower transverse thread first extends under two successive lower longitudinal threads, then extends above a lower longitudinal thread, extends again under two successive lower longitudinal threads, and then extends above a lower longitudinal thread; see for example the lower longitudinal thread 42. Every lower transverse thread extends above two other lower longitudinal threads, so that each lower longitudinal thread is run over by a lower transverse thread in the lower pattern repeat. Here, the lower fabric is formed with three shafts in the transverse direction, i.e. the course of the respective lower transverse thread recurs after three longitudinal threads.

In the exemplary embodiment shown in FIG. 6, the lower fabric is bound six times to the upper fabric within the total repeat, namely once by each upper transverse thread. In this respect, each lower longitudinal thread is bound once in the total repeat. A stable layer connection and a fine, homogeneous paper side may be achieved at the same time with the total repeat according to FIG. 6 as well.

With the screen according to the invention, for example with a screen having a total repeat according to FIG. 5 or FIG. 6, numerous advantageous features may be achieved or at least are made possible, amongst others the following features:

-   -   a high fineness of the paper side for providing an appropriate         or rather high fiber support,     -   a high uniformity of the paper side for reducing the tendency to         marking of the screen,     -   a machine side with a high degree of openness for achieving         appropriate or rather good draining properties and a reduced         tendency to congestion of the fabric with fibers and impurities,     -   a machine side with a high mechanical stability, for example         against expansion,     -   prevention or reduction of interior wear, whereby a separation         of the layers can be avoided/postponed, so that the lifetime of         the screen is long.

In other words, a fine upper fabric with a homogeneous design can be used for sheet forming and bound to a more stable and coarser lower fabric. In this respect, the upper fabric is designed with a weave that is comparable to a plain weave in terms of its sheet forming properties and is therefore excellently suited for producing graphical paper. The lower fabric may be configured to be coarser in the longitudinal and transverse directions than the upper fabric, and may therefore provide the following advantages: solid matter potentially having entered the fabric/screen through the upper fabric is not/hardly kept in the lower fabric; the draining properties of the screen are solely determined by the paper side or rather the upper fabric, i.e. the lower fabric is configured in such an open manner that its influence on the total flow is subordinate. Thus, according to the invention, the coarse lower fabric may be used for optimizing the screen with respect to mechanical stability, resistance to wear and fabric thickness.

Thus, a very fine and homogenous paper side fabric layer is achieved (e. g. when compared to the machine side), whereas the machine side fabric layer may be designed with a higher degree of openness and relatively thick longitudinal and transverse threads which may contribute to a high mechanical strength. At the same time, a particularly stable connection of the upper fabric layer and the lower fabric layer is made possible; generally by a weft or transverse thread-bound design of the screen and e. g. by means of a particular design of the paper side/upper transverse threads with a binding area/zone. In this respect, the upper transverse threads each have a course that corresponds in sections to a plain weave, with binding areas being generated, in which the respective upper transverse thread extends under four successive upper longitudinal threads. These binding areas may be used for binding the lower fabric layer to the upper fabric layer. Thereby, a high binding frequency and hence a high stability of the screen can be achieved. For example, the binding frequency may be increased by up to 33% using the screen according to the invention—when compared to known screens—by using the same number of threads and the same mathematical fiber support index (FSI according to Beran). 

1. Paper machine screen, formed as a multi-layer fabric, having an upper fabric layer and a lower fabric layer, wherein the upper fabric layer comprises a plurality of upper longitudinal threads which extend solely in the upper fabric layer, and a plurality of upper transverse threads which are interwoven with the upper longitudinal threads and each extend at least predominantly in the upper fabric layer, wherein the lower fabric layer comprises a plurality of lower longitudinal threads which extend solely in the lower fabric layer, and a plurality of lower transverse threads which are interwoven with the lower longitudinal threads and extend solely in the lower fabric layer, wherein the upper fabric layer has a recurring upper pattern repeat on its upper side, which is formed by nine upper longitudinal threads and three upper transverse threads, wherein the three upper transverse threads of the upper pattern repeat each comprise the following course in the upper pattern repeat: under an upper longitudinal thread, over the next upper longitudinal thread, under the next upper longitudinal thread, over the next upper longitudinal thread, under the four next successive upper longitudinal threads and over the following upper longitudinal thread, wherein in the upper pattern repeat the course of the respective upper transverse thread is arranged to be offset with respect to the course of the neighboring upper transverse thread by three upper longitudinal threads in a transverse direction, and wherein in the fabric at least some of the upper transverse threads are formed as connecting transverse threads which temporarily change into the lower fabric layer during their course under the four successive upper longitudinal threads for connecting the upper fabric layer with the lower fabric layer, and there extend under at least one lower longitudinal thread.
 2. Paper machine screen according to claim 1, wherein the lower longitudinal threads are greater in diameter than the upper longitudinal threads, and/or wherein the lower transverse threads are greater in diameter than the upper transverse threads.
 3. Paper machine screen according to claim 1, wherein the ratio of upper longitudinal threads to lower longitudinal threads in the entire fabric is 3:2 or 1:1, and/or wherein the fabric comprises or consists of a recurring total repeat in which the ratio of upper longitudinal threads to lower longitudinal threads is 9:6 or 9:9.
 4. Paper machine screen according to claim 1, wherein the ratio of upper transverse threads to lower transverse threads in the entire fabric is 2:1 and/or wherein the fabric comprises or consists of a recurring total repeat in which the ratio of upper transverse threads to lower transverse threads is 6:3 or 12:6.
 5. Paper machine screen according to claim 1, wherein at least one of the three upper transverse threads of each upper pattern repeat is formed as a connecting transverse thread which temporarily changes into the lower fabric layer during its course under the four successive upper longitudinal threads and there extends at least under one lower longitudinal thread, in order to connect the upper fabric layer to the lower fabric layer.
 6. Paper machine screen according to claim 5, wherein at least two of the three upper transverse threads of each upper pattern repeat are formed as a connecting transverse thread.
 7. Paper machine screen according to claim 1, wherein the respective connecting upper transverse thread extends under exactly one lower longitudinal thread or exactly two successive lower longitudinal threads during its course under the four successive upper longitudinal threads for connecting the upper fabric layer with the lower fabric layer.
 8. Paper machine screen according to claim 1, wherein the connection of the upper fabric layer and the lower fabric layer occurs exclusively by means of the upper connecting transverse threads.
 9. Paper machine screen according to claim 1, wherein the upper fabric layer exclusively consists of a plurality of abutting upper pattern repeats and/or wherein the respective upper pattern repeat exclusively consists of the nine upper longitudinal threads and the three upper transverse threads.
 10. Paper machine screen according to claim 1, wherein the lower transverse threads are interwoven with the lower longitudinal threads in such a manner that they form a three-shaft or six-shaft weave in the lower fabric layer, in which the course of the respective lower transverse thread recurs after three and six longitudinal threads, respectively.
 11. Paper machine screen according to claim 1, wherein the longitudinal threads are formed as warp threads and the transverse threads are formed as weft threads.
 12. Paper machine screen according to claim 2, wherein the ratio of upper longitudinal threads to lower longitudinal threads in the entire fabric is 3:2 or 1:1, and/or wherein the fabric comprises or consists of a recurring total repeat in which the ratio of upper longitudinal threads to lower longitudinal threads is 9:6 or 9:9.
 13. Paper machine screen according to claim 2, wherein the ratio of upper transverse threads to lower transverse threads in the entire fabric is 2:1 and/or wherein the fabric comprises or consists of a recurring total repeat in which the ratio of upper transverse threads to lower transverse threads is 6:3 or 12:6.
 14. Paper machine screen according to claim 2, wherein at least one of the three upper transverse threads of each upper pattern repeat is formed as a connecting transverse thread which temporarily changes into the lower fabric layer during its course under the four successive upper longitudinal threads and there extends at least under one lower longitudinal thread, in order to connect the upper fabric layer to the lower fabric layer.
 15. Paper machine screen according to claim 2, wherein the respective connecting upper transverse thread extends under exactly one lower longitudinal thread or exactly two successive lower longitudinal threads during its course under the four successive upper longitudinal threads for connecting the upper fabric layer with the lower fabric layer.
 16. Paper machine screen according to claim 2, wherein the connection of the upper fabric layer and the lower fabric layer occurs exclusively by means of the upper connecting transverse threads.
 17. Paper machine screen according to claim 2, wherein the upper fabric layer exclusively consists of a plurality of abutting upper pattern repeats and/or wherein the respective upper pattern repeat exclusively consists of the nine upper longitudinal threads and the three upper transverse threads.
 18. Paper machine screen according to claim 2, wherein the lower transverse threads are interwoven with the lower longitudinal threads in such a manner that they form a three-shaft or six-shaft weave in the lower fabric layer, in which the course of the respective lower transverse thread recurs after three and six longitudinal threads, respectively.
 19. Paper machine screen according to claim 2, wherein the longitudinal threads are formed as warp threads and the transverse threads are formed as weft threads. 